Up to the present time, there have been many proposals of inventions which measure the wall thickness of an object of interest using a gamma ray wall thickness gauge and set and correct the rolling conditions based on the measured results (see Patent Document 1, for example).
A gamma ray wall thickness gauge measures wall thickness based on the amount of attenuation of gamma rays passing through an object of interest. On account of this principle of measurement, it is not possible to measure the wall thickness of a hollow shell into which a mandrel bar has been inserted using a gamma ray wall thickness gauge. Therefore, it is of course not possible to measure the wall thickness between the stands of a mandrel mill with a gamma ray wall thickness gauge. Even on the exit side of a mandrel mill, thickness measurement can only be performed with a retractable mandrel mill in which the inserted mandrel bar is retracted towards the entrance side after the completion of elongation rolling. Moreover, even with a retractable mandrel mill, the wall thickness can be measured only in a location spaced by a certain amount from the immediate vicinity of the exit of the mill. Due to these limitations, there is naturally a limit to the extent to which the results of wall thickness measurement by a gamma ray thickness gauge can be utilized to carry out high accuracy control.
Thus, in a control method for a mandrel mill using a conventional gamma ray wall thickness gauge, there is the fundamental problem that it is not possible to measure the wall thickness of a hollow shell into which a mandrel bar is inserted. Due to this problem, the following four specific problems exist.
(Problem 1)
In a conventional control method using a gamma ray wall thickness gauge, in initial elongation rolling, it is not possible to set a roll gap corresponding to the outer diameter of the mandrel bar, and thus a high accuracy cannot be obtained for the wall thickness of hollow shells which initially undergo elongation rolling.
A method which is conceivable in order to obtain a hollow shell with a highly accurate wall thickness by elongation rolling using a mandrel mill is one in which the outer diameter of a mandrel bar is estimated by calculation and the roll gap of a predetermined stand is set in accordance with the estimated outer diameter of the mandrel bar. In the past, in order to carry out this method, the wall thickness of a hollow shell on the exit side of a mandrel mill (a retractable mandrel mill) was measured using a gamma ray wall thickness gauge, and based on the results of this measurement and the set value of the roll gap in the final stand, the outer diameter of the mandrel bar was estimated. However, this estimating method estimates the outer diameter of a mandrel bar based on the measured value of the wall thickness on the exit side of a mandrel mill. Therefore, elongation rolling of a hollow shell for which the wall thickness was measured is already completed by the time that the outer diameter of the mandrel bar is estimated. A mandrel mill normally uses a plurality of mandrel bars while circulating them, and a highly accurate wall thickness cannot be obtained for the first hollow shell to be rolled using each mandrel bar, i.e., for the same number of hollow shells as the number of mandrel bars being circulated,
(Problem 2)
As the measurement of wall thickness of a hollow shell by a gamma ray wall thickness gauge cannot be performed between stands of a mandrel mill, its wall thickness facing the flange portions wall thickness must be predicted, thereby making it impossible to obtain a highly accurate wall thickness.
The locations of a hollow shell corresponding to its flange portion wall thickness at one stand becomes locations corresponding to its groove bottom wall thickness at the next stand where the wall thickness is reduced by grooved rolls. Accordingly, if an error develops in the prediction of the flange portion wall thickness of a hollow shell, an error develops in the amount of reduction. In addition, the speed of the hollow shell on the entrance and exit sides of the stand varies, and the tensile force between stands varies. As a result, the deformation of the hollow shell in the locations corresponding to the flange portion wall thickness which occurs at the next stand greatly deviates from the prediction, and defective rolling with worsened dimensional accuracy occurs with a hollow shell made of a difficult-to-work material.
(Problem 3)
In elongation rolling using a mandrel mill, opposing thickness deviations, which are a phenomenon in which thick-walled portions and thin-walled portions alternatingly develop at a pitch of approximately 90° in the circumferential direction of a hollow shell, sometimes develop. In order to suppress the occurrence of opposing thickness deviations, the rolling positions of the grooved rolls can be adjusted so that the thick-walled portions become thinner and the thin-walled portions become thicker. However, as stated above, with a gamma ray wall thickness gauge, it is not possible to carry out wall thickness measurement except on the exit side of a mandrel mill (a retractable mandrel mill) and in a location separated from the mill by a certain extent. Therefore, even if opposing thickness deviations develop in a hollow shell which underwent wall thickness measurement, it is no longer possible to adjust the rolling positions of the grooved rolls for that hollow shell. In addition, in a mandrel mill of a type other than a retractable mandrel mill, it is not possible at all to perform measurement of opposing thickness deviations.
(Problem 4)
In elongation rolling using a mandrel mill, it is important to ascertain the distance between the groove bottoms of grooved roll in a stand. Since this distance cannot be directly measured, the rolling position is corrected by zero adjustment of rolling position achieved by contacting the flange portions of the grooved roll with each other in view of the result of measurement by a gamma ray wall thickness gauge installed on the exit side of the mandrel mill. However, only the rolling position of the grooved rolls installed in the final stand can be corrected by this method. Accordingly, in a control method for a mandrel mill using a conventional gamma ray wall thickness gauge, it is not possible to increase the accuracy of zero adjustment of the rolling position of the grooved rolls in previous stands.
Patent Document 1: JP H08-71616 A1